(a) Field of the Invention
The present invention relates to a method for fabricating a semiconductor optical device having a ridge stripe and, more particularly, to an improvement of its optical characteristics.
(b) Description of a Related Art
A semiconductor laser device having a ridge stripe for confinement of emitted laser light and injection current therein has excellent laser characteristics as well as the advantage of a simple structure, and thus is used as pump sources for fiber amplifiers in the optical communication system, or light source for optical recording system and optical measurement system.
For fabrication of the semiconductor laser device having the ridge stripe, a heterojunction structure of semiconductor layers is formed on a semiconductor substrate, followed by formation of the ridge stripe at the top portion of the semiconductor layers. Thereafter, an insulator film is formed on the entire surface of the semiconductor layer structure and selectively removed at the top of the ridge stripe. After forming electrodes at the top and bottom portion for current injection, a semiconductor laser device having the semiconductor layer structure is obtained.
FIGS. 1A to 1H show consecutive steps of a conventional process for fabricating a ridge stripe in a semiconductor laser device.
A heterojunction semiconductor layer structure 10 formed on a semiconductor substrate 11 has therein an active layer 12 including one or more of quantum well active layer, and has a ridge stripe with a double channel structure at the top portion. That is, the semiconductor layer structure includes ridge stripe 14 at the top thereof and a pair of channels 13 at both sides of the ridge stripe 13, as shown in FIG. 1A. A a thin insulator film 15 is then formed on the entire surface, as shown in FIG. 1B.
A first resist film 16 is formed thereon by spin-coating to a specified thickness, which is thicker than the height of the ridge stripe 14 with respect to the channels 13, as shown in FIG. 1C, followed by a heat treatment to obtain an insoluble property thereof. A second resist film 17 is then formed by spin-coating on the first resist film 16, as shown in FIG. 1D.
Thereafter, the second resist film 17 is subjected to a patterning using a photolithographic and etching process including exposure and development steps. Thus, the second resist film 17 is etched selectively from the first resist film 16, to form a mask 17 having an opening 18 therein above the top of the ridge stripe 14, as shown in FIG. 1E. Subsequently, the first resist film 16 is subjected to an oxygen-plasma ashing process for selectively removing the first resist film 16 at the opening 18 by using the mask 17, as shown in FIG. 1F.
The insulator film 15 is then removed by using reactive io etching (RIE) at the bottom of the opening 18 to expose the top of the ridge stripe 14, as shown in FIG. 1G. The first and second resist films 16 and 17 are then entirely removed using a plasma ashing process to expose the remaining portion of the insulator film 15. The top surface of the ridge stripe 14 is generally implemented by a contact layer having a function for assuring electric contact with an electrode to be formed thereon.
In the conventional process as described above, two different resist films of a positive image type having different viscosity coefficients are used for the first and second resist films 16 and 17.
In the etching process for the insulator film 15, there is a problem in that the insulator film 15 is sometimes removed at both side surfaces of the ridge stripe 14 in spite of the fact that the insulator film 15 is desired to cover the side surfaces of the ridge stripe while exposing the top of the ridge stripe 14.
The reason of the problem is that the first resist film 16 is in fact etched to some extent at the top of the ridge stripe 14 due to the poor reproducibility or controllability in the conditions or amount of the exposure light in the photolithographic and etching process for the second resist film 17. This causes the portion of the first resist film 16 covering the ridge stripe 14 to have a thinner thickness or to have small openings at the sides of the ridge stripe 14. This in turn causes the oxygen-plasma ashing for removing the first resist film 16 to damage the insulator film 15 or etch the insulator film 15 at the side surfaces of the ridge stripe 14, as shown in FIG. 1G. The exposure of the side surfaces of the ridge stripe 14, especially exposure of the sides of the upper cladding layer in the ridge stripe 14, causes degradation of the laser characteristics. Particularly, the degradation due to the COD (catastrophic optical destruction) is a crucial problem for AlGaAs material system.
The exposure of the side surfaces of the ridge stripe 14 occurs especially in the case of a ridge stripe having a narrow width as small as 6 xcexcm or less, due to the alignment error of the mask with respect to the ridge stripe. This may be alleviated to some extent by using a fine alignment of the mask, which, however, consumes a large time length and reduces the process efficiency.
Patent Publication JP-A-6-1349 describes a plurality of layered resist films having different solubility coefficients, which may assure a safe etching of the second resist film 17 selectively from the first resist film 16 in the above process. However, in our experiments, this technique was not effective in the above process due to the presence of the ridge stripe 14, which caused different thicknesses in the first resist film on the top of the ridge stripe 14 depending on the viscosity coefficient thereof after the spin-coating. The different thicknesses in turn caused a residual portion of the first resist film after the oxygen-plasma ashing for the first resist film, which reduced yield of the products.
As described above, the conventional process does not offer a satisfactory solution for the problem that the insulator film is etched at the side surfaces of the ridge stripe to expose the upper cladding layer, and thus a semiconductor optical device having a ridge stripe has poor characteristics. In addition, the selective etching of the second photoresist film is not conducted with a satisfactory productivity or process efficiency
In the above description, the problem of a semiconductor laser device is exemplified. However, the above problem is common to other semiconductor optical devices having a ridge structure, such as a ridge waveguide photodetector or a semiconductor amplifier.
In view of the above, it is an object of the present invention to provide a method for fabricating a semiconductor optical device having a ridge stripe and excellent device characteristics at a relatively low cost or high process efficiency.
The present invention provides a method for fabricating a semiconductor optical device including the steps of: forming a layer structure including semiconductor active layer on a semiconductor substrate; forming a ridge stripe on the semiconductor layer structure; forming an insulator film on the semiconductor layer structure including the ridge stripe; forming on the insulator film a first photoresist film of a negative image type having a viscosity of 50 centipoises or less; forming a second photoresist film on the first resist film; patterning the first and the second photoresist films to have therein an opening for exposing a portion of the insulator film at a top of the ridge stripe; removing the portion of the insulator film exposed by the opening; and forming an optical device having the semiconductor active layer, the patterning step including development using a liquid developer to which the first and the second resist films have different solubility coefficients.
In accordance with the method of the present invention, the configurations including the different solubility coefficient of the first and second photoresist films with respect to the liquid developer, and the first photoresist film having a negative image type and a viscosity of 50 centipoises or less can afford excellent patterning of the second photoresist film without etching of the first photoresist film in the vicinity of the top surface of the ridge stripe during exposure and development steps for the second photoresist film.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.